StackOverflow2013

Note that there are some explanatory texts on larger screens.

plurals

POExistentials and Scrap your Boilerplate
primarykey
Id
2839713
data
AcceptedAnswerId
2840978
AnswerCount
1
ClosedDate
CommentCount
0
CommunityOwnedDate
CreationDate
2010-05-15T10:16:06.037
FavoriteCount
0
LastActivityDate
2010-05-23T08:32:59.183
LastEditDate
2010-05-23T08:32:59.183
LastEditorUserId
24044
OwnerUserId
24044
ParentId
0
PostTypeId
1
Score
8
ViewCount
338
LastEditorDisplayName
text
Body
I'm writing a XML (de)serializer using Text.XML.Light and Scrap your Boilerplate (at <a href="http://github.com/finnsson/Text.XML.Generic" rel="nofollow noreferrer">http://github.com/finnsson/Text.XML.Generic</a>) and so far I got working code for "normal" ADTs but I'm stuck at deserializing existentials. I got the existential data type <pre><code>data DataBox where DataBox :: (Show d, Eq d, Data d) => d -> DataBox </code></pre> and I'm trying to get this to compile <pre><code>instance Data DataBox where gfoldl k z (DataBox d) = z DataBox `k` d gunfold k z c = k (z DataBox) -- not OK toConstr (DataBox d) = toConstr d dataTypeOf (DataBox d) = dataTypeOf d </code></pre> but I can't figure out how to implement <code>gunfold</code> for <code>DataBox</code>. The error message is <pre><code>Text/XML/Generic.hs:274:23: Ambiguous type variable `b' in the constraints: `Eq b' arising from a use of `DataBox' at Text/XML/Generic.hs:274:23-29 `Show b' arising from a use of `DataBox' at Text/XML/Generic.hs:274:23-29 `Data b' arising from a use of `k' at Text/XML/Generic.hs:274:18-30 Probable fix: add a type signature that fixes these type variable(s) </code></pre> It's complaining about not being able to figure out the data type of <code>b</code>. I'm also trying to implement <code>dataCast1</code> and <code>dataCast2</code> but I think I can live without them (i.e. an incorrect implementation). I guess my questions are: <ol> <li>Is it possible to combine existentials with Scrap your Boilerplate?</li> <li>If so: how do you implement gunfold for an existential data type?</li> </ol>
Tags
<generics><reflection><haskell>
Title
Existentials and Scrap your Boilerplate
singulars
PostAcceptedAnswerId
1. PO
 singulars
 PostTypePostTypeId
 PTAnswer
PostParentId
1. This table or related slice is empty.
PostTypePostTypeId
1. PTQuestion
UserLastEditorUserId
1. USfinnsson
UserOwnerUserId
1. USfinnsson
plurals
PostLinksPostIdRelatedPostId
1. This table or related slice is empty.
PostLinksRelatedPostIdPostId
1. This table or related slice is empty.
PostsAcceptedAnswerId
1. This table or related slice is empty.
PostsParentIdCreationDate
1. PO
 singulars
 PostTypePostTypeId
 PTAnswer
VotesPostIdCreationDate
1. VO
 singulars
 PostPostId
 POExistentials and Scrap your Boilerplate
 UserUserId
 This table or related slice is empty.
 VoteTypeVoteTypeId
 VTUpMod
2. VO
 singulars
 PostPostId
 POExistentials and Scrap your Boilerplate
 UserUserId
 This table or related slice is empty.
 VoteTypeVoteTypeId
 VTUpMod
3. VO
 singulars
 PostPostId
 POExistentials and Scrap your Boilerplate
 UserUserId
 This table or related slice is empty.
 VoteTypeVoteTypeId
 VTUpMod
CommentsPostId
1. This table or related slice is empty.

Querying!

Guidance

A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.